1. Field
The present disclosure relates to the mechanical actuation of valves in fluidic devices. In particular, a method and apparatus are disclosed for the mechanical actuation of valves in flexible, fluidic devices for the regulation of fluid flow.
2. Description of Related Art
PDMS (poly-dimethylsiloxane) microfluidic devices have enabled inexpensive rapid prototyping of sophisticated microfluidic applications (Unger, et al., 200, Science, 288:113-116; Thorsen, et al., 2002, Science, 298:58-584). Due to incompatibilities with many solvents, acids, and bases, PDMS is not suitable for many applications in chemistry. However, several solvent-resistant elastomers have been shown to be suitable for functional microfluidic device fabrication, and can be used as a replacement for PDMS in certain applications (Rolland et al., 2004, JACS, 126: 2322-2323; van Dam, R. M. Solvent-Resistant Microfluidic Devices and Applications, PhD Thesis, California Institute of Technology, August, 2005).
Elastomeric devices in the art are constructed from two layers and a substrate as shown on the left in FIG. 1 (Studer et al, Journal of Applied Physics 95(1), 2004, pg 393-398). The fluid layer contains “fluid channels” (50), and the control layer (300) contains “control channels”. Devices can also be fabricated with the control channel above the fluid channel; however, the configuration shown on the right in FIG. 1 results in valves with lower actuation pressures and allows more design flexibility in the fluid layer. FIG. 1 shows the operation of pneumatically/hydraulically actuated micro valves in which the control channels are pressurized to cause deflection of the thin elastomer membrane separating the control channel from the channel where they cross. Deflection of the control channel into the fluid channel obstructs the flow, thus acting as a valve.
One way to make off-chip connections (i.e. connections between the devices and off-chip components such as fluid reservoirs, waste containers, chromatography columns, and pressure supplies) is by punching holes through the elastomer before bonding to the substrate. Tubing (typically stainless steel) is then inserted directly into the holes. If the tubing is slightly larger than the hole, the tubing is held in place by the friction, enhanced by the elasticity of the device material, which squeezes around the tubing.
To actuate a valve, the pressure in the control channel (300)(FIG. 1) should be sufficient both to deflect the valve membrane material and overcome the pressure in the fluid channel (50). In some applications, for example, those involving evaporation of solvents such as water or acetonitrile, the fluid pressure can become quite high (30 psi or more, depending on temperature). The control channels should thus be pressurized to even higher pressures, leading in some cases to device failure by delamination of device layers (peeling apart), or rupture of device material. In PDMS chips designed for the synthesis of FDG (2-deoxy-2-[18F]fluoro-D-glucose), an additional problem is presented when the device layers made of PDMS are bonded by plasma treatment. Such bonding is weakened under basic conditions. Since the first step of FDG synthesis involves evaporation of a K2CO3 solution to dryness, it leads to the device delamination at much lower pressures than the chips can normally withstand. Furthermore, at high pressures, failures can occur at connections to off-chip pressure sources. These problems are often exacerbated in solvent-resistant materials.
Therefore, what is needed is a new method and apparatus for the mechanical actuation of a valve in fluidic devices (FIG. 1 on the right) made from flexible materials that in the least eliminates both high pressure within the chip and the problems associated with the layer bonding of solvent-resistant materials found in the prior art.